In the previous discussion of migration algorithms (Part 1), their shortcomings and strengths were presented in the context of their implementation with a known velocity field. It is easy to imagine that if the velocity model is inaccurate, then a reliable portrait of the subsurface cannot be developed. In Part 2, we will touch the key element of successful imaging, particularly for prestack depth migration (PSDM), the accuracy of the velocity field.

The velocity field for imaging is developed from the seismic data itself. This should come as no surprise when we consider that the task of imaging is essentially...